the nearest planes are the ones you take as references for your impedance
calculation: the electromagnetic wave guided by your line is bounded by the
firsts planes it founds, and on them (it, in the case of a microstrip) induces
the return current.

You want a good decoupling among reference planes in order to have them AC-short
circuited, which minimize the mismatch (inductance) between a driver referenced
to a certain plane, a line referenced to another plane and/or a charge
referenced to another plane.

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